Energy-filtered imaging is being used to map the organization of DNA in specific regions of nuclei by imaging the phosphorus distributions in cultured cells that are labeled with fluorescently tagged antibodies to reveal the location of transcriptionally active genes. Plastic embedded sections are analyzed with an FEI CM120 electron microscope equipped with a post-column imaging filter and a 1024x1024 pixel cooled slow-scan CCD camera. Elemental maps are computed from images recorded below and above the phosphorus L23 core edge by careful subtraction of the pre-edge background intensity. High-pressure freezing techniques are being investigated to determine whether improved ultrastructure can be obtained from DNA fibers within the nucleus in order to localize the transcription sites with higher precision.